The present invention relates to a method for recovering substantially all CO2 generated in a combustion process which includes an application of a mixed conducting membrane. Furthermore, the present invention relates to use of this method.
Due to the environmental aspects of CO2, the possibilities for reducing the emissions of this compound to the atmosphere from combustion processes has been widely discussed.
Conventional combustion processes, used for carbon containing fuels and where the oxygen source is air, having carbon dioxide concentrations of 3-15% in the combustion products, hereinafter called exhaust gas, dependent on the fuel and the applied combustion- and heat recovery process. The reason the concentration is this low is because air is made up of about 78% by volume of nitrogen.
Thus, a reduction in the emission of carbon dioxide to the atmosphare makes it necessary to either separate the carbon dioxide from the exhaust gas, or raise the concentration to levels suitable for use in different chemical processes or for injection and deposition in e.g. a geological formation for long term deposition or for enhanced recovery of oil.
CO2 can be removed from cooled exhaust gas, normally discharged off at near atmospheric pressure, by means of several separation processes e.g. chemical active separation processes, physical absorption processes, adsorption by molecular sieves, membrane separation and cryogenic techniques. Chemical absorption for instance by means of alkanole amines is e.g. considered as the most practical and economical method to separate CO2 from exhaust gas. The separation processes, however, require heavy and voluminous equipment and will consume a substantial amount of heat or power. Applied in a power generation process, this process will reduce the power output with around 10% or more.
An increase of the concentration of CO2 in the exhaust gas to levels suitable for use in different chemical processes or for injection and deposition e.g. in a geological formation for long term deposition or for enhanced recovery of oil from an oil reservoir is possible by burning fuel in pure oxygen instead of air.
Commercial air separation methods (e.g. cryogenic separation or pressure swing absorption (PSA)) used to produce pure oxygen require 250 to 300 KWh/ton oxygen produced. Supplying oxygen e.g. to a gas turbine by this methods will decrease the net power output of the gas turbine cycle by at least 20%. The cost of producing oxygen in a cryogenic unit will increase the cost of electric power substantially and may constitute as much as 50% of the cost of the electric power.
However, a less energy demanding methode than the separation methods mentioned above is known from European patent application 0658 367-A2 which desribes an application of a mixed conducting membrane which is integrated with a gas turbine system by heating air in a gas turbine combuster and further by selective permeation of oxygen through the membrane. Pure oxygen near atmospheric pressure or below and at high temperature is recovered from the permeate side of the conducting membrane. An oxygen partial pressure difference causes oxygen to be transported through the membrane by reduction of oxygen on the high oxygen partial pressure side (rententate side) and oxidation of oxygen ions to oxygen gas on the low oxygen partial pressure side (the permeate side). In the bulk of the membrane oxygen ions are transported by a diffusion process. Simultaneously the electrons flow from the permeate side back to the feed side of the membrane.
Application of a sweep gas in combination with a mixed conducting membrane to lower the oxygen partial pressure to increase the degree of oxygen removal or oxygen recovery is known from the U.S. Pat. No. 5,562,754. In this patent a method for combined production of oxygen and power is disclosed by heating air in a gas turbine combuster and by selective permeation of oxygen through the membrane. In order to improve the efficiency of gas separation by the membrane, the permeate side of the membrane is swept by e.g. steam supplied for instance from the heat recovery section of the power plant. The sweep gas is heated in a separate high temperature heat exchanger. The application of sweep gas will reduce the partial pressure of oxygen on the permeate side of the membrane and thereby increase the flux of oxygen through the membrane. However, this require a certain amount of sweepgas and therefore a certain energy amount to generate this sweep gas. This will therefore decrease the net power output of the power generating process.
Application of a sweep gas in combination with a mixed conducting membrane is also known from Norwegian patent application NO-A-972632 (published Jul. 12, 1998). This patent describes a power and heat generating process where a fuel is combusted with an oxidant, which is an O2/CO2/H2O-containing gaseous mixture, which is supplied from a mixed conducting membrane. The oxygen is picked up from the permeate side of the mixed conducting membrane by means of a sweep gas. The sweep gas is the product or part of the product from at least one combustion process upstream the membrane. In this patent application the sweep gas, or part of the sweep gas, containing a mixture of mainly CO2 and H2O also act as the working fluid in a gas turbine cycle. The amount of sweep gas is related to the amount of working fluid required in the gas turbine cycle i.e. to control the temperature in the gas turbine combuster. Working fluid is the gas (oxidant and fuel) transported through the gas turbine system. Air fed to the retentate side of this membrane is heated by combusting a fuel in the air stream in a burner.
To obtain a sufficient high flux of oxygen through the membrane a rather high temperature is required (600-1500xc2x0 C.). On the air side of the membrane this may be accomplished by combusting a fuel in the air stream in a burner to increase the temperature of the air fed to the membrane, for instance as disclosed in European patent application 0658 367-A2 or as described in Norwegian patent application NO-A-972632 (published Dec. 7, 1998). The most convenient and least expencive method is to use a carbon containing fuel, e.g. a fossil fuel. However, by means of this method the heated air stream will contain CO2 generated in the bumer. The CO2 concentration in the oxygen depleteded air stream discharged from the retentate side of the membrane will be less than about 10% and in most cases less than 3%. If recovery of all generated CO2 in a combustion process is desirable, due to environmental aspects of CO2, an oxygen depleted air stream containing low CO2 -concentrations is not desirable.
Application of a staged mixed conducting membrane process is known from U.S. Pat. No. 5,447,555 which describes a method for producing pure oxygen. In this process high purity oxygen is recovered from air by a high-temperature ion transport membrane system comprising two or more stages in which each stage operates at a different feed side to permeate side pressure ratio. Operation of the system in multiple stages at controlled pressure ratios produces oxygen at a lower specific power consumption compared with single-stage operation. Sweep gas is not used in this US patent.
The main object of this invention was to arrive at an energy efficient method to recover substantially all CO2 generated in a combustion process.
The described object can be fulfilled by application of a method which include an application of a mixed conducting membrane.
Hot steam or a mixture of steam and CO2 (e.g. recycled exhaust gas) is used as sweep gas to pick up oxygen on the permeate side of a mixed conducting membrane (MCM) in a first stage. The membrane is capable of separating oxygen from a hot air stream fed to the retentate side of the membrane. Sweep gas now containing oxygen is applied as oxidant in a catalytic or non catalytic combustion process where a carbon containing fuel is combusted. Heat generated in the combustion process is used to heat air fed to the retentate side of the membrane.
The hot combustion products, i.e. the exhaust gas, containing CO2, H2O and a low concentration of O2 is used as sweep gas in a second MCM stage and the concentration of oxygen in the sweep gas is increased in the second membrane stage to a sufficiently high level to be used as oxidant in a second combustion stage. Heat generated in the second combustion process is also used to heat air to the MCM-process. Hot combustion products leaving the second combustion stage is used as sweep gas to pick up more oxygen in a third MCM-stage to be used as oxidant in a third combustion stage. The number of required combustion stages and MCM-stages depends on the amount of sweep gas fed to MCM-stage one and on the required pre-heating temperature of air to the retentate side of the MCM-process.
The oxygen produced in the membrane is removed between each stage by combustion with fuel in a combuster. The partially cooled CO2 containing exhaust gas with a low concentration of oxygen is used as sweep gas in the next MCM stage.
This will reduce the amount of sweep gas necessary for production of a given amount of oxygen and thus reduce the size of equipment necessary for producing sweep gas to the first MCM stage. Application of e.g. 10 stages will reduce the amount of sweep gas with about 95% compared with a single stage process and reduce the energy required to generate sweepgas in the same order of magnitude.
Air fed to the retentate side of each mixed conducting membranes is heated by heat exchanging with hot exhaust gas generated in at least one combuster.
If sweep gas is not generated during the process or used as a working fluid in a gas turbine cycle the sweep gas has to be generated in a separate process. If sweep gas is generated in a separate process the cost of sweep gas is related to the required amount of sweep gas. The cost of sweep gas generation will be reduced if the amount of sweep gas used in the air heating process is reduced. In a one stage mixed conducting membrane process this reduced amount of sweep gas will, however, reduce the rate of oxygen transport through the membrane. This will further increase the required membrane area and thus the membrane costs. Otherwise the sweep gas pressure has to be reduced. This will, however, increase the pressure drop of oxygen through the membrane and thus reduce the efficiency of the heat generating process.
In the present invention each stage operates at nearly the same pressure and the staged process will not increase the membrane area requirement. Since oxygen is removed between each stage the driving forces for transport of oxygen through the membrane will increase and reduce the membrane area requirement and the costs.
The problems mentioned above concerning reduced transport of oxygen, increased costs or reduced efficiency, if the amount of sweep gas, which may be steam or a mixture of steam and/or recycled exhaust gas, is reduced, is solved by application of the staged combined mixed conducting membrane and combustion process described in the present invention.
In order to avoid excessive temperatures in the combustion process, comprising use of catalytic or non catalytic combusters, the exhaust gas which is used as sweep gas in a subsequent mixed conducting membrane stage, is cooled between the stages by heat exchanging with air to generate hot air. Furthermore the temperature in the combuster stages are controlled by varying the concentration of oxygen in the sweep gas.
In order to obtain a sufficient high flux of oxygen through the mixed conducting membranes a high air temperature is required which is achieved by the heat exchanging method described above according to the present invention. The air stream is heated in several stages in heat exchangers located between the membrane stages or is divided into several streams and each stream is heated in a heat exchanger located between two membrane stages.
Heated air generated by the method according to the present invention can be used to generate pure oxygen in a mixed conducting membrane.
Furthermore, heated air generated by the method according to the present invention can be used to generate synthesis gas consisting of one or more of the components CO, CO2, H2 and N2 or for generating heat in a mixed conducting membrane reactor where the membrane reactor is capable of reacting a mixture of steam and a carbon containing fuel with oxygen permeated through the said membrane to make synthesis gas and/or heat.
Further, the method according to the present invention is used in a heat and/or power generating process.
Further, the CO2-containing exhaust gas generated by the method according to the present invention is used for enhanced oil and natural gas recovery or for injection in a gelogical formation or is used in a chemical process to make carbon containing products. Oxygen eventually left in the CO2 containing combustion gas exit in the last combustion stage can be removed in a catalytic oxidation reactor or in a combined mixed conducting membrane and partial oxidation reactor as described in patent application NO-A-972631 (published Jul. 12, 1998).